Inorganic particles, e.g., those containing silicate, have been used as fillers for composite materials, e.g., printed wiring boards and encapsulating materials, to improve their dimensional accuracy, heat resistance and so forth. Crushed glass fibers are sometimes used as inorganic particles because they have uniform compositions and can be easily treated to reduce proportions of coarse particles. However, crushed glass fibers tend to be columnar often with high length/diameter ratios, and hence to have increased viscosity, when contained as fillers in resins in a high proportion, at resin melting point to greatly deteriorate resin flowability. Hence, they have limited applications as fillers to be contained in high proportions. Moreover, their dispersibility in resins is far from sufficient.
For production of inorganic particles with uniform size distributions, Patent Document 1, for example, discloses a process for producing glass powder containing particles of excessively high or low aspect ratios to limited extents by pressing crushed glass fibers in a container. This technique, however, is difficult to give spherical particles, because they are broken under a mechanical pressure where they lie over one another. Hence, they are far from sufficient as fillers which can keep resins well fluid when they are incorporated therein in high proportions.
Patent Document 2 discloses a process for producing fine glass particles of sharp size distribution by crushing glass fibers after they are coated with tetraalkoxy silane. This technique is also difficult to give spherical particles, as is the case with the technique disclosed by Patent Document 1, and are far from sufficient as fillers which can keep resins well fluid when they are incorporated therein in high proportions.
For production of spherical metal oxide particles, Patent Document 3 discloses a process in which metal powder, e.g., that of metallic silicon, as a starting material is thrown into a chemical flame to form metal particles in an atomized state, which are exploded to synthesize the ultrafine oxide particles, e.g., silicon dioxide particles. This process, however, involves danger and high cost.
For production of spherical particles from glass fibers, Patent Document 4, for example, discloses a process for treating fibrous glass as a starting material, in which glass fibers coated with an organic material, e.g., binder, is thrown into a flame, after being crushed, to spheroidize the vitreous substance while burning up the organic coating material. This process, however, is difficult to completely burn up the organic substance, and tends to allow the spherical particles produced to reagglomerate with each other and turn into composite particles, because cooling is not adopted positively.
This technique, however, is originally developed to produce raw glass powder for recycling, in which spent fibrous glass coated with an organic substance is recycled to be reused as industrial raw materials. The glass powder produced by this process ultimately needs treatment of the organics or the like to produce industrial products.
For production of spherical glass particles, Patent Document 5 discloses a process in which molten glass as a raw material is ejected from a nozzle into a continuous flow moving downwards to turn into glass droplets, which are deorbited from a downward path to be recovered in a recovery tank.
For production of spherical glass particles, Patent Document 6 discloses a process in which a raw glass material is thrown into an oxygen-containing plasma to be molten and spheroidized.
For production of spherical glass particles, Patent Document 7 discloses a process in which a solution or sol containing a varying mixture as a raw material for glass, whose composition is adjusted to have a very specific final glass composition, e.g., that containing SiO2 at 2 to 15% by mass, is ejected into a flame and the resulting fine droplets are quenched. The product of this process can have specific purposes, e.g., production of inter-layer insulation films for fluorescent displays for photolithography and thin glass films as substrates.
Patent Document 8 discloses a process for production of spherical glass particles, substantially similar to that disclosed by Patent Document 7, except that the final glass composition is controlled to contain SiO2 at 40 to 70% by mass, to improve flowability of a resin in which they are incorporated. The spherical glass particles as the final product can have specific purposes, e.g., production of thin glass films as substrates, as is the case with that produced by the Patent Document 7 process.
On the other hand, Patent Document 9 discloses a composition for high-strength glass fibers containing SiO2, Al2O3 and MgO at specific contents and substantially free of an alkali metal, which are produced in a short melting time and contain substantially no hollow fibers.
For treatment of glass fibers with an acid, Patent Document 10 discloses a process for treating E-glass fibers with an aqueous acidic solution to form a surface layer containing silica at a high content and thereby to improve heat resistance of the fibers without sacrificing the E-glass fiber characteristics.
Also for treatment of glass fibers with an acid, Patent Document 11 discloses a process for producing glass cloth of a specific E-glass composition, in which glass cloth of E-glass composition is immersed in an aqueous acetic acid solution to elute an alkaline-earth and alkali metals into the solution and thereby decrease their content to 20 ppm or less based on the glass cloth.
For treatment of silicon dioxide particles with an acid, Patent Document 12 discloses a process in which metallic silicon particles are burnt in an oxygen-containing flow to form the silicon dioxide particles having an average diameter of 0.01 to 10 μm, and the resulting particles are washed with a low-concentration aqueous mineral acid solution, e.g., nitric acid, to remove radioactive element compounds, e.g., uranium compounds.
For treatment of metallic silicon particles, Patent Document 13 discloses a process in which metallic silicon particles are washed with an aqueous mineral acid to remove radioactive elements, e.g., uranium and thorium, to 1 ppb or less, and the treated particles are burnt in an oxygen-containing flow to produce high-purity silicon dioxide particles having an average diameter of 0.01 to 10 μm.    Patent Document 1: JP-A-4-338133    Patent Document 2: JP-A-2003-192387    Patent Document 3: JP-B-1-55201    Patent Document 4: JP-A-11-228164    Patent Document 5: JP-A-2005-179145    Patent Document 6: JP-A-2004-338961    Patent Document 7: JP-A-8-310836    Patent Document 8: JP-A-8-91874    Patent Document 9: JP-A-11-21147    Patent Document 10: JP-A-7-172876    Patent Document 11: JP-A-2001-73253    Patent Document 12: JP-B-7-61855    Patent Document 13: JP-B-7-61856